1. Field of the Invention
The present invention relates to a magnetic recording medium, and more particularly to a high density recording medium having excellent physical and electromagnetic conversion characteristics.
2. Description of the Prior Art
As ferromagnetic powders which have been employed for magnetic recording media, there are, for instance, .gamma.-Fe.sub.2 O.sub.3, Co-containing .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-containing Fe.sub.3 O.sub.4 and CrO.sub.2. However, the magnetic characteristics of these ferromagnetic powders, such as the coercive force and the maximum residual magnetic flux density, are inadequate for high density recording with high sensitivity, and they are not suitable for magnetic recording of a signal having a short recording wavelength of at most about 1 .mu.m or for magnetic recording with a narrow track width.
As the requirements for such magnetic recording media have become to be severe, ferromagnetic powders having characteristics suitable for high density recording have been developed or proposed. As such magnetic powders, there may be mentioned metals or alloys such as Fe, Co, Fe-Co, Fe-Co-Ni and Co-Ni, or alloys of these materials with Al, Cr or Si. A magnetic recording layer employing such an alloy powder is required to have a high coercive force and a high residual magnetic flux density for the purpose of the high density recording, and it is required to select a process for the preparation or an alloy composition so that the above-mentioned magnetic powder meets these requirements.
The present inventors have prepared magnetic recording media by employing various alloy powders, and have found that a magnetic recording medium with a noise level being sufficiently low and suitable for high density short wave recording is obtainable when the specific surface area as measured by BET method is at least 48 m.sup.2 /g, the coercive force of the magnetic layer is at least 1000 Oe, and the surface roughness of the magnetic layer is at most 0.08 .mu.m as a R.sub.20 value (an average value of 20 values as measured by Talystep method of Taylor-Hobson Co. by means of a needle of 0.1.times.2.5 .mu.m under a needle pressure of 2 mg and with a cut off of 0.17 mm).
On the other hand, magnetic recording media having a thin ferromagnetic metal layer formed on a plastic base film have recently been used for the same purpose of high density recording.
A thin ferromagnetic metal layer formed by electroplating, chemical plating, vacuum vapor-deposition, sputtering or ion plating, is composed of 100% metal or alloy, and thus is capable of having a high recording density. However, when a thin ferromagnetic metal layer is formed by such a method, the surface condition of the substrate gives a substantial effect on the surface condition of the thin ferromagnetic layer, and thus affects the electromagnetic conversion characteristics, although the surface roughness can be thereby minimized to a level of 0.01 .mu.m.
Besides, in either one of these magnetic recording media, there is a trend that the thickness of the substrate such as a plastic base film made of e.g. polyethylene terephthalate, polyethylene naphthalate, polyimide or polyamide, becomes thinner. At present, a thickness of 11 .mu.m or less is being studied. As the base film becomes thinner, the medium tends to be so flexible that the friction will increase, and the tape running tends to be difficult. Therefore, it has been desired to improve the stiffness of the base film and thereby to regain the good running property. As a method for improving the running property, it has been proposed to provide a top coating on the magnetic surface. In such a case, however, there has been a problem such that the lubricating agent on the top coating is not durable, or gives rise to stickiness during storage under a high temperature condition. Further, when a top coating is provided, the surface roughness of the magnetic layer is minimum, whereby adhesion due to the tightening of the tape winding is likely to result, such being undesirable.
It has been common to provide a backing layer instead of the top coating, to ensure smooth running, proper winding and adequate tape strength. However, the properties of the conventional magnetic tapes provided with a backing layer are not necessarily satisfactory, and the following problems have been pointed out.
(a) A decrease of the S/N ratio as a result of providing the backing layer.
(b) Occurrence of a cinching phenomenon due to an inclusion of air.
(c) Abrasion of the backing layer.
(d) A magnetic tape for a long hour operation being susceptible to a damage during the loading or unloading operation.
(e) Defective winding of a magnetic tape.